Osmolal balance after open intracardiac operations in children.

756-761. Changes in plasma osmolality, urine osmolality, urine volume, total osmolal output, and free osmolal output have been studied in children between 2 and 10 years of age. All underwent elective open intracardiac operations. Plasma hypo-osmolality develops after operation, and this was not influenced by altering the osmolal concentration of the prime in the extracorporeal system nor was it affected by severe water restriction in the immediate postoperative period. During the first 48 hours a water diuresis was not seen even in the presence of marked plasma hypo-osmolality. Urine volume, urine osmolality, and solute concentration in the kidney are affected by both the perfusate prime and postoperative fluid regime. As the total osmolal output was the same in all study groups, there was an inverse relation between urine volume and urine osmolality. The data obtained show no evidence of postoperative renal dysfunction. Free osmolal output, a measure of solute and water excretion, showed a normal renal response to a solute load but an abnormal response to a water load. The data support the use of moderate haemodilution in the perfusate prime, the precise osmolal concentration not being of major importance. Postoperatively a regime of moderate fluid restriction is supported.